Since energy saving and environmental protection currently become the main focus in the development of automobile industry, hybrid vehicles have been turned into a main core technology and pursued by many automobile manufactures globally. Wherein, the solution of plug-in hybrid vehicles, except pure electric drive vehicles, is one of the most fuel saving solutions, and it has being advocated by many major automobile manufacturers. A plug-in hybrid vehicle primarily consists of a relative lower displacement engine and one or two motors. Under normal conditions, the motor(s) is responsible for providing pure electric power output and braking recycling energy when the power battery energy and power in relative high, achieving engine starting when the power battery energy and power drop to a preset value, and generating electricity or directly involving in the power drive, and the like.
In the current existing technology, the engine and the motor(s) in the above-mentioned plug-in type hybrid drive system are coupled mostly by the following ways:
I. Solution with planetary gear for power coupling and electric variable transmission for shifting: For example, Volt hybrid concept vehicle provided by GM is a plug-in hybrid electric vehicle, and the batteries in the hybrid drive system can be charged by using the outlet of household power supply (e.g. 110V/220V). The plug-in hybrid vehicle can provide a longer pure electrically-driven travelling distance than a full hybrid electric vehicle. The power coordination and variable speed can be realized by connecting the engine and two motors by means of a planetary gear set, and using torque spliting principle, so as to meet the demand of the running of the whole car. The change of the speed ratio is realized by controlling the speed of the integrated starter-generator. However, this method is complex in power transmission path, and there is a phenomenon that power transmission loss is caused unnecessarily by converting mechanical energy into electric energy and then converting the converted electric energy into mechanical energy to output. Therefore, if this method is applied for a plug-in hybrid vehicle, it will lead to the loss of efficiency of the hybrid mode of the hybrid drive vehicle. In addition, since all output power is output by planetary gear carrier of the planetary line, the requirements for the costs on material and the manufacturing of the planet gear carrier are very high.
II. Solution with single clutch for power coupling and without shifting: F3DM type hybrid vehicle is launched by BYD Holding Ltd., in which a relatively simple connection manner is used (referring to CN patent application No. 200610141069.1, entitled “DRIVE DEVICE FOR HYBRID VEHICLE”). An engine and a starting-generating integrated machine are linked together and then they are connected with another main motor via a clutch and connected directly to the input shaft of a differential via a main decelerator. This method is simple in structure, and under ideal operational condition power is transmitted directly and therefore the transmission efficiency is high. However, under the condition that a vehicle is in pure electric drive, it is very difficult to meet all design requirements in terms of climbing, acceleration performance and maximum electric travelling speed simultaneously, because the vehicle is driven directly by the main decelerator. In general, a large torque and high speed motor must be employed, that makes the demands on functions of electronic (electric) power controller and battery system for matching with the motor increase accordingly. Even so, it is very difficult to ensure the motor could often operate in a higher efficiency region under the urban traveling condition. In addition, in case when the energy of battery is low, so it is required to change into motor/engine hybrid travelling mode, but due to restrictions on minimum stable rotation speed and torque characteristics of engine, the engine will only be able to operate in a way like that in the above-mentioned series mode in many travelling conditions, such as driving at intermediate or low speed on urban road and ramp road, thus reducing energy utilization efficiency greatly.